art and craft of nano-photonics materials

Antimony trisulfide inverted opals grown by CBD

This is the
first inverse opal made from a chalcogenide whose optical properties should lead
to the achievement of a PBG in the VIS. The selected material is antimony
trisulfide, Sb2S3.

The mineral, with an orthorhombic
crystalline structure, is known as stibnite. The absorption edge of Sb2S3
thin films lies in 2.2 eV (564 nm) for the amorphous form and in 1.78 eV (697
nm) for the crystalline state. In the transparency region the value of the
dielectric constant varies from 9.6 to 10.9 in the amorphous material and from
13.6 to 14.4 in the crystalline state. In both cases it is high
enough to obtain a PGB. In the first case, in the VIS range
from 564 nm and in the second one, in the NIR from 700 nm.

Several methods have been reported
to grow Sb2S3 thin films such as chemical bath
deposition (CBD) in non-aqueous and aqueous media, vacuum evaporation spray
pyrolisis,and flash evaporation as well as others to obtain precipitated
powders. None of the recipes based on CBD methods, provides successful results
in the infiltration of opal matrices. Most authors use complexing agents like
Oxalic or Tartaric acids to control the growth of thin films and avoid the fast
precipitation resulting from direct mixing of two solutions containing Sb3+
cations and S2- anions. Following the CBD methods, thin films of Sb2S3
grew on the opal surface that prevented further growth in the internal
interstices. In addition, trying to grow the material in consecutive steps or
baths, dissolves the previously synthesised material. On one hand, Sb2S3
is dissolved in a new SbCl3 aqueous medium probably due to the
protonation of the sulfur ion to give H2S. On the other hand, it is a
well-known fact that stibnite can be dissolved in media containing sulfide ions
resulting in hydrosolubles anionic species.

Here we demonstrate that
precipitation of amorphous Sb2S3 followed by subsequent
annealing provides stable enough structures to withstand silica etching and lead
to inverted structures. In this work we have obtained infilling ratios of up to
ca. 100% of the pore volume. The process assisted by a suction system begins
with the precipitation of amorphous antimony trisulfide. Although CBD is often
performed using a single bath we used two different Sb3+ and S2-
ions containing baths, antimony trichloride and thioacetamide
aqueous solutions, respectively. No extra agents were used to complex the
antimony source. Sb2S3mm)
integrated in a funnel under vacuum. The opal was first placed on the porous
plate and soaked up with 0.5 mL SbCl3 (35 wt%) solution (pH ≈ 1) at
room temperature while suction was applied. Then, after drying the opal surface
(in air) the same was done with the thioacetamide solution (2.0 M) at 55ºC (pH
»
5). Finally, the opal was heated at 65ºC for 20 minutes. After that the opal
looked orange due to the amorphous nature of the inorganic Sb2S3. was grown by a method of
precipitation in aqueous solution in which the opal was placed on a pyrex porous
plate (porosity around 100-120

It is worth mentioning that other reagents,
concentrations and solvents were explored. For the antimony source, SbCl3
was used not only in an aqueous medium, but also dissolved in acetone, ethylene
glycol, acetic acid and diluted sulphuric acid to yield antimony trisulfide
under different solvent effects on the dissociation of SbCl3. Sources
of antimonyl ions as antimony (III) acetate were also tested. For sulfide ions
sodium thiosulfate and thiourea were among the alternatives to the thiacetamide
solution.

By means of the precipitation
method, several opals with different sphere diameters were loaded with Sb2S3.
Various techniques were applied to characterize the samples: Raman spectroscopy,
Scanning Electron Microscopy (SEM) including Energy Dispersive X-ray analysis (EDX)
and VIS-NIR spectroscopy.

An opal with a sphere diameter of 380 nm filled with amorphous Sb2S3
is shown in figures 1a) and 1b). The as-prepared material is amorphous and
presents a granulated look.
Further improvement in the optical properties of the composites was attained
upon crystallization by annealing
the amorphous synthesized material. This
process provides an increase in the dielectric constant of Sb2S3.
The annealing treatment was carried out in nitrogen atmosphere at 325ºC for 2
hours in a tubular furnace.

Fig. 2.- Different orientations of a
stibnite inverse opal. a) and b) show (111) facets. c) and d) show the (100)
ones.

As the final step, to obtain an
inverse opal and achieve a higher ν, the sample was
dipped in a fluorhidric acid solution (1 wt. %) during 7 hours. By means of this
procedure, the silica spheres were removed resulting in a new structure made of
interconnected air spheres. SEM images of (111) and (100) surfaces of an
inverted structure are shown in fig.2. The crystalline form shows a smoother
appearance (fig.2) than the granulated amorphous material (fig.1).

To ascertain the quality of the
amorphous and crystalline Sb2S3, a characterization of the
material has been performed using Raman spectroscopy with the 514.5 nm line of
an Ar+ laser. In fig. 3 the spectra of amorphous and crystalline
stibnite synthesized here are shown and compared with commercial standard.
Fig.3.a) shows the Raman spectrum of amorphous Sb2S3 in a
silica-amorphous Sb2S3 composite and fig.3.b that of one
crystallized and inverted. The former presents a broad peak characteristic of
amorphous materials. The Raman spectra of commercial stibnite is also plotted
(fig.3.c) and results to be completely identical to the one synthesized in this
work.

The elemental composition of the
inverted opals was studied by Energy Dispersive X-Ray analysis (EDX). The
absence of a Si Ka peak in the EDX
spectrum reveals that the chemical etching of the composite has been complete
and all silica has been removed in the inverted opal. Additionally, the only
detected peaks in the EDX spectrum, shown in fig.4, are assigned to S and Sb. In
fig.4, the EDX spectrum of a sample of Sb2S3 standard has
also been included for comparison. A single peak at 2.3 keV is evidenced that is
assigned to S Ka while a group of four
peaks located in the region of higher energy is assigned to Sb La.
These EDX spectra were recorded from large sample areas representing,
consequently, an average elemental composition of the antimony sulfide. The
relative intensity of S and Sb peaks in the studied and reference samples are
identical suggesting that a stoichiometric sulfide is present in the inverted
opal. Additionally, different point analysis and line profiles of the S Ka and Sb Ka, not shown,
revealed no significant changes in composition indicating that the studied
sample is homogeneous.

Additional characterization of the
material was performed by X-Ray powder diffraction technique to verify the
quality of the Sb2S3 grown. A broad peak in the
diffraction pattern indicated the amorphous nature of as grown material. Upon
crystallization of the material, the diffraction pattern showed narrow and
well-defined peaks, in good accord with those of the standard.

Fig. 5. Reflectance spectra from a 585 nm sphere diameter bare opal (dotted
line) and the same opal loaded with crystalline Sb2S3 up
to a 73% of the pore volume.

Optical spectroscopy, employed for
the photonic band structure characterization, was carrier out with the use of a
Fourier Transform Spectrometer (Bruker IFS 66) to which a microscope was
attached. The focalization and collection of reflected light was performed with
a 36× Cassegrain microscope objective. With this set up incidence and
reflectance is collected for all angles between 20º and 57º which implies
internal angles between 5º and 12º.

Optical
reflectance measurements were carried out not only for the mere purpose of
characterization in various stages of fabrication but to analyse the photonic
properties of the opal-based structures. All the peaks described here correspond
to features in the band structure. For a given lattice constant (determined by
the spheres diameter) the positions of the peaks are mainly determined by the
average dielectric constant, eav,
and their widths are related to n. Antimony trisulfide infiltrated opals present a
broad and well defined reflectance peak at low energies corresponding to first
order Bragg diffraction. This peak is much broader than that of the host bare
opal and red shifted with respect to it. The magnitude of this shift permits to
assess the amount of infiltration attained by knowing the dielectric functions
of silica and stibnite and the initial silica filling fraction, which is
precisely controlled by the previous sintering of the host opal.

Fig
6.-Reflexion spectra from crystalline Sb2S3-silica opal
along with the band structures for 80% (a) and 100% (b) infilled opal. The
marked regions highlight the correspondence between gaps in the band structure
and the peaks in the spectra.

The reflectance measurements shown
in fig. 5 present an example for a 585 nm diameter silica spheres opal. The size
of the beads was selected to avoid placing the relevant photonic bands features
in the electronic gap of the material. As can be seen in the spectra, the bare
opal shows a Bragg peak centred at 1.26 mm.
After opal infiltration and annealing procedures the peak of the crystalline
composite is shifted to 1.88 mm. The
width is also larger due to the increase in ν.

In order to understand the
photonic band gap properties of the materials, full band gap structure
calculations were performed for the different systems to be compared with the
experimental results. In fig. 6 and following, reflectance spectra are plotted
next to the band structure of the corresponding system to allow for direct
comparison. Only the first 20 bands are plotted. In particular for the SiO2-Sb2S3
composite (fig. 6) two representative spectra are plotted along with
calculations assuming infiltration degrees of 80% and 100%. Although the
material synthesized is very uniform in composition the infiltration is less
uniform and can vary from point to point of the sample in a few percent. Owing
to the high dielectric function of the guest material small variations in
infiltration can lead to strong energy variations of the different band
structure features. Although the two spectra shown here are qualitatively
similar a good assignment of the peaks can only be done in the assumption of
such composition variations. The three peaks found are very plausibly assigned
to the three gaps that open in or around the L point in the Brillouin zone. The
magnitude and position of these gaps vary differently on departing from L toward
different neighbouring points among which the most representative of the L
symmetry are W and K, that being the reason for the path chosen. The geometry of
the experimental set up determines the range of wave vectors probed to be around
half way between L and K or L and W. For that reason the arrows included in the
diagrams are located approximately in the middle of the relevant gaps. The close
correspondence between gaps and peaks supports the assignment.

Fig
7.-Reflexion spectrum from crystalline Sb2S3 inverted opal
and the band structure for a 100% infilled region.

For heavily infiltrated opals a
process of inversion can be carried out by dissolution of the SiO2 in
fluorhidric acid. The resulting structure has a much higher dielectric contrast
and, as a consequence, bands flatten out and gaps widen. This is shown in fig.
7. In particular, the second and third reflectance peaks from the composite, all
but merge in a single broad feature. This has a small dip in the centre that can
be accounted for by what can be seen as a bunch of bands isolated in the middle
of a broad gap. The high energy part of this feature corresponds to the full gap
that this structure presents. A display of the whole band structure shows that,
in effect, this gap spans the whole Brilloun zone so that the peak observed
would be present regardless of the direction in which reflectance was probed.